1. Field of the Invention
The present invention is generally related to luminescent transition metal complexes, and more particularly to blue-emitting organometallic complexes and their application.
2. Description of the Prior Art
Phosphorescent organic light emitting diodes (OLEDs) are under intensive investigation because of their potential of achieving improved device performances. The electrophosphorescence are easily generated from both singlet and triplet excited states and, thus, their internal quantum efficiency can reach a theoretical level of unity, rather than the 25% inherent upper limit imposed by the formation of singlet excitons for their fluorescent counterparts. Thus, a great deal of effort has been spent on the third-row transition metal complexes, for developing highly efficient phosphors that can emit all three primary colors: red, green and blue. Despite of the elegant research on both red and green phosphors, there are only scatter reports on the room temperature blue phosphors. The best known example is one Ir(III) complex named FIrpic (as shown in FIG. 1) which has proved to be an excellent dopant for sky-blue phosphorescent OLEDs. Further improvements were made by substituting picolinate with other ancillary ligands such as pyridyl azolate to afford FIrtaz (as shown in FIG. 1) and even employing a combination of cyanide and phosphine. These modifications have produced a hypsochromic shift of ˜10 nm versus the emission of FIrpic. However, significant lowering of quantum efficiency was noted in some cases, which have hampered the fabrication of the true-blue phosphorescent OLEDs. Therefore, new blue light-emitting materials are still needed corresponding to increasing phosphorescent performance and practical operation durability.